Eberron: Dawn of Heroes
Half-elves share the continent of Khorvaire with the other common races. When the elves first came to the continent and began mingling with the human settlers, the first half-elves were born. Over time, the half-elves gathered and formed two distinct family groups. These groups that eventually became House Lyrandar and House Medani. For two thousand years, the half-elves of Khorvaire have possessed a culture and society of their own. Occasionally, elves and humans still produce half-elves, but those belonging to the two half-elf dragon-marked houses breed true as a race of their own.